Firewall

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A firewall is typically some software which separates the local net from the Internet. If you couple a LAN to the Internet, the common network protocol must be TCP/IP. This means that every single server and work station on the net is assigned an IP address.

So when a PC is on the Internet with its own IP address it is visible to other net users. In this mode the confidential data from the local net become available to others, unless you set up protection against this. A firewall will among other things make the local net IP addresses invisible from the outside.